1. Field of the Invention
The present invention is in the field of driving tools for application of helical thread inserts into tapped apertures adapted to receive the same.
2. The Prior Art
Helical thread inserts are becoming increasingly used as a means for repairing stripped, worn or damaged threads formed in castings or bores, as well as in original equipment applications wherein it is desired to provide a threaded bore which is particularly resistant to damage as a result of frequent insertion and removal of threaded studs or the like. Often the tapped threads of a bore in a casting or like fixture which may be fabricated of relatively soft metal will disintegrate in use, whereby the threads will no longer afford secure mounting for a bolt or stud.
It is conventional practice to repair such castings by drilling out the bore to remove remnants of the damaged threads, thereafter retapping the bore and inserting into the tapped bore a helical insert, which may be of hardened stainless steel or the like. The outer diameter of the insert intimately engages the newly formed threads in the bore, the inner diameter of the insert corresponding to the diameter and thread pitch presented by the original tapped aperture in the bore.
In order to assure that the helical insert is fixedly positioned within the retapped bore, the helix is formed of a larger diameter than the bore and is driven into position, utilizing substantial torque. The diameter of the insert after driving is less than the diameter of the insert in its unstressed condition, the radial outward forces exerted by the applied insert assuring the frictional retention of the same in the bore.
Driving of the insert is effected by a mandrel which heretofore had comprised an elongate hardened metal cylinder having a driving end and a torque transmitting end. An example of such mandrel is disclosed in U.S. Pat. No. 3,348,293.
Mandrels of the type disclosed in the above noted patent required frequent replacement for the reasons more fully set forth in my U.S. Pat. No. 4,077,101, which patent refers to an improved drive mandrel incorporating a polymeric body portion whereby the torquing forces required for application of an insert were cushioned by the polymeric material, resulting in increased life for the tool and reduced instances of cross-threading or misapplication of the insert.
I have determined that although the effective life of the tool of my aforesaid patent is generally greater than tools heretofore used, the considerable torsional forces required to be exerted in the driving of an insert into position when applied through a polymeric driver member as shown and described in my aforesaid United States patent, particularly where the inserts and mandrel are of small diameter, are such as in time to rupture or destroy portions of the threaded end of the mandrel, reducing the effective life of the tool.